Femtosecond Raman-Induced Kerr Effect Study of Temperature

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Femtosecond Raman-Induced Kerr Effect Study of TemperatureDependent Intermolecular Dynamics in Imidazolium-Based Ionic Liquids: Effects of Anion Species and Cation Alkyl Group Shohei Kakinuma, Tateki Ishida, and Hideaki Shirota J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11009 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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The Journal of Physical Chemistry

Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Imidazolium-Based Ionic Liquids: Effects of Anion Species and Cation Alkyl Group

Shohei Kakinuma,† Tateki Ishida,§ and Hideaki Shirota†‡*

†

Department of Nanomaterial Science and ‡ Department of Chemistry, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan

§

Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan

[email protected]

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ABSTRACT The temperature dependence of the intermolecular vibrational dynamics in imidazolium-based ionic liquids (ILs) with ten different anions was studied by femtosecond Raman-induced Kerr effect spectroscopy. For all the ILs investigated in this study, the intensity in the low-frequency region below 50 cm−1 increases and the spectral density in the high-frequency region above 80 cm−1 decreases (and shows a red shift) with increasing temperature. The first phenomenon would be attributed to the activation of the translational vibrational motions, while the second one is ascribed to the slowing librational motion of the imidazolium ring with increasing temperature. Calculated spectra of the density of states for the intermolecular vibrations of 1butyl-3-methylimidazolium hexafluorophosphate, which is one of the experiment samples studied here, obtained by molecular dynamics simulation agreed well with the experimental results and confirmed the spectral assignments.

When we compare the difference spectra

between spectra measured at various temperatures and spectrum measured at 293 K, a clear difference was found in the ~50 cm-1 region of the Kerr spectra of 1-butyl-3-methylimidazolium thiocyanate and 1-butyl-3-methylimidazolium dicyanamide from those of the other ILs. The difference might be originating from the librational motions of the corresponding anions. We also compared the temperature dependent Kerr spectra of hexafluorophosphate salts of 1-butyl-3methylimidazolium, 1-hexyl-3-methylimidazolium, and 1-heptyl-3-methylimidazolium cations. These ILs showed the similar temperature dependence that was not affected by the alkyl group length. The temperature dependent viscosities and glass transition temperatures of the ILs were also estimated to determine their fragilities.


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1. INTRODUCTION Intermolecular dynamics, such as collision (or interaction)-induced motion, librational (caging) motion, and collective orientational (structural or α-) relaxation, in liquids and solutions appears in the frequency range below ~200 cm-1. Because the intermolecular dynamics affects the elementary processes of chemical reactions in solutions, a deep understanding of the intermolecular dynamics in liquids and solutions is crutial.1 After the appearance of stable femtosecond solid lasers in the 1980’s, a wide variety of ultrafast molecular spectroscopic techniques has been developed at the laboratory level, e.g., femtosecond Raman-induced Kerr effect (also called optical Kerr effect) spectroscopy (fsRIKES)2,3 and terahertz time-domain spectroscopy (THz-TDS).4 These techniques have allowed recording spectra in a low-frequency region and with a higher signal-to-noise ratio compared to the conventional steady-state Raman and IR spectroscopic methods. Because fs-RIKES and THz-TDS detect Raman-active modes and IR-active modes, respectively, they are complementary spectroscopic techniques for the low-frequency region. In view of the wider detection frequency range of fs-RIKES than THz-TDS, studies performed on the low-frequency spectra of liquids and solutions based on the former are more abundant than those performed by the latter technique. The low-frequency spectra and the intermolecular dynamics in molecular liquids studied by fs-RIKES are described in several excellent reviews and in-depth articles.5-10 Recently, fs-RIKES has been applied to more complicated systems, such as solvents in nonporous glasses,11-14 polymer liquids15 and solutions,16-19 microemulsions,19,20 biomolecules,2125

and others.26



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Ionic liquids (ILs) are molten salts at room temperature,27,28 attracting the attention of scientists, researchers, and engineers because of their intriguing properties. For example, the negligible vapor pressure under ambient temperature and pressure conditions, the ability to dissolve various solutes (including polar, weakly polar, and ionic compounds), and the high electrical conductivity make ILs promising greener solvents for applications such as chemical reactions, fuel cells, and batteries.29-31 It is reasonably understood that the unique properties of ILs result from their microscopic intermolecular interactions and structures, affecting their intermolecular vibrations significantly. In view of this, the low-frequency spectra and the intermolecular dynamics of ILs have been investigated by several techniques, such as steady-state Raman spectroscopy,32-34 far-IR,35-37 THz-TDS,38-41 and, of course, fs-RIKES.40-50 Very recently, the low-frequency spectra of as many as forty aromatic cation based ILs were collected using fs-RIKES to overview the features of the intermolecular vibrational dynamical features of imidazolium- and pyridinium-based ILs.51

To understand the detailed molecular-level aspects of the low-frequency dynamics,

molecular dynamics (MD) simulations were also performed for comparison with experimental data.52-55 Some studies comparing complementary experimental and simulated results were also reported,56-63 being very successful in revealing the molecular-level aspects of ILs. Among the studies of the low-frequency region in ILs by fs-RIKES, the number of those dealing with temperature dependence is limited. Quitevis and coworkers reported the first fsRIKES study of the temperature dependent low-frequency spectra of 1-methyl-3pentylimidazolium ([C5MIm]+)-based ILs with bis(trifluoromethylsulfonyl)amide ([NTf2]−).64 Subsequently, the authors also reported the corresponding results for bromide (Br−) and hexafluorophosphate ([PF6]−) anions.65 According to their results, the temperature dependence 4 Environment ACS Paragon Plus

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of the low-frequency spectrum of [C5MIm][NTf2] is different from those of the other ILs. Sonnleitner et al. compared the temperature-dependent low-frequency dielectric and Kerr spectra of ethylammonium nitrate and propylammonium nitrates in a broad frequency range from ~0.002 to 500 cm-1,66 showing the structural relaxation component is more temperature-sensitive than the intermolecular vibrational band. We also studied the temperature dependence of the lowfrequency spectra of [NTf2]−-based ILs with monocations (1-methyl-3-propylimidazolium and 1hexyl-3-methylimidazolium) and dications (1,6-bis(3-methylimidazolium-1-yl)hexane and 1,12bis(3-methylimidazolium-1-yl)dodecane) by RIKES.67

The above study showed that the

intensity in the low-frequency region below 20 cm−1 increases with increasing temperature, whereas the high-frequency component with a peak at ~80 cm−1 shifts to a lower frequency side with rising temperature for both the monocationic and dicationic ILs. In this study, we focus on the effects of anions species on the temperature dependence of the low-frequency spectra of ILs. The anions were selected on the basis of their molecular structures and categorized into the following three types (Figure 1): (i) spherical top anions (hexafluorophosphate ([PF6]−), tetrafluoroborate ([BF4]−), and iodide (I−)), (ii) less symmetric anions (trifluoromethansulfonate ([OTf]−), thiocyanate ([SCN]−), dicyanamide ([DCA]−), and cyclohexafluoropropane-1,3-bis(sulfonyl)amide ([NCyF]−)), and (iii) anions having two stable conformers (bis(fluorosulfonyl)amide ([NF2]−), bis(trifluoromethylsulfonyl)amide ([NTf2]−), and bis(pentafluoroethylsulfonyl)amide ([NPf2]−)).

1-Butyl-3-methylimidazolium ([C4MIm]+),

which is one of the most popular IL cations, was used as the counter cation. We also examined the effects of alkyl group length on the temperature-dependent low-frequency spectra by comparing the results obtained with a common anion, [PF6]−, for 1-hexyl-3-methylimidazolium ([C6MIm]+) and 1-heptyl-3-methylimidazolium ([C7MIm]+) to that of [C4MIm]+. Furthermore,



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the temperature-dependent low-frequency spectra of some molecular liquids that are analogous to the cations or anions of ILs, namely, 1-methylimidazole, propionitrile, and CCl4 were investigated in order to see how the nature of ILs (consisting only of ions) affects the temperature dependence of their spectra. To understand the temperature-dependent behavior of ILs, the temperature dependence of the density of states (DOS) for intermolecular vibrations of [C4MIm][PF6] was examined by MD simulations. In addition to the low-frequency spectra, we also estimated the temperature-dependent viscosities, the glass transition temperatures, and the melting points for the samples in this study.

2. Experimental and Computational Methods 2.1. Materials. [C4MIm][PF6] (Fluka), [C4MIm]I (Kanto), [C4MIm][OTf] (Iolitec), [C4MIm][DCA] (Merck), [C4MIm][NF2] (Kanto), [C4MIm][NTf2] (Kanto), 1-methylimidazole (Aldrich), propionitrile (TCI), and CCl4 (Wako) were used as received. [C4MIm][SCN] was received from Aldrich

and

subsequently

purified

by

column

chromatography

(aluminum

oxide,

chloroform/ethanol, 20:1 v/v) after the standard activated charcoal decolorization treatment.68 [C4MIm][BF4], [C4MIm][NPf2], and [C6MIm][PF6] were previously synthesized,51 and the preparation details were already reported. [C4MIm][NCyF] and [C7MIm][PF6] in this study were prepared in our laboratory according to previously reported preparation procedures for imidazolium-based ILs.69 The above synthesized samples were identified by 1H HMR and elemental analyses. The estimated values of carbon, hydrogen, and nitrogen contents of the ILs agreed with the calculated values within ±0.4% that is the criteria in the Journal of Organic


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Chemistry. Detailed synthetic procedures and assignments for these ILs are described in the Supporting Information. The water contents of ILs were measured to be less than 100 ppm using a Karl Fischer titration coulometer (Hiranuma, AQ-300), as are summarized in Supporting Information. The sample ILs were dried in vacuo for more than 36 hours prior to measurements. Because the liquid property values of the ILs estimated in this study are in good agreement with the reported values (see below), these values were not influenced much by the water content.

2.2. Viscosity and Glass Transition Temperature. The viscosities (η) of the ILs and molecular liquids used in this study were measured at various temperatures (278 – 353 K at 5 K interval) using a reciprocating electromagnetic piston viscometer (Cambridge Viscosity, ViscoLab 4100) equipped with a circulating water bath (Yamato, BB300). The glass transition temperatures (Tg) of the ILs were measured using a differential scanning calorimeter (DSC, TA Instruments DSC 2920).

IL samples of 12–18 mg were

hermetically sealed in aluminum pans, cooled to approximately −153 K, and then heated to 323 or 373 K (depending on the sample melting point (Tm)) to erase their thermal histories. They were again cooled to approximately −153 K and subsequently heated to 323 or 373 K. The values of Tg and Tm were determined in the second heating cycles at a scan rate of 5 K/min.

2.3. fs-RIKES



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The fs-RIKES setup used in this study was constructed in our laboratory, with details described elsewhere.10,70 Briefly, a titanium sapphire laser (KMLabs Inc., Griffin) pumped by a Nd:YVO4 diode laser (Spectra Physics, Millennia Pro 5sJ) with the output power of 4.25 W was used as the light source in the current RIKES setup.71 The output power of the titanium sapphire laser was approximately 400 mW.

The typical temporal response was estimated as

approximately 37 ± 3 fs (full width at half maximum) from the cross-correlation between the pump and probe pulses using a 200 µm thick KDP crystal (type I). Scans with a time resolution of 4096 points at 3.335 fs/step were performed for a time window of 12.3 ps. Long time window data are often collected to observe the (picosecond) overdamped relaxation process, but only short time window data were acquired in this study, because its primarily purpose was the investigation of intermolecular vibrational dynamics and not picosecond overdamped relaxations, and the number of studied samples was large. The adequacy of the discussion based on the spectra obtained from only the short time window data (without long time window data over hundreds of picoseconds) has been confirmed in our previous study.67 Pure heterodyne signals were realized by combining the Kerr transients recorded at both ~+1.5° and ~−1.5° rotations of the input polarization using a quarter-wave plate to eliminate the residual homodyne signal. Three scans were averaged for each polarization measurement.

Prior to the fs-RIKES

measurements, the sample liquids were injected into a 3 mm optical path length quartz cell (Tosoh Quartz) using a 0.02 or 0.2 µm pore-size Anotop filter (Whatman). The samples in a cell were then dried in vacuo for more than 36 hours and sealed by a Teflon cap tightly prior to measurements. It should be noted that the Kerr spectrum of an imidazolium based IL (1-methyl3-octylimidazolium tetrafluoroborate) with the water mole fraction of less than 0.6 does not change much by addition of water.72 The temperature of the samples during the fs-RIKES


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measurements was kept constant by a laboratory-built temperature controller based on a Peltier temperature controller set (VICS, VPE35-5-20TS).

2.4. Computational Details MD simulations were carried out with the GROMACS 5.0.6 simulation package.73 All the simulations utilized the force field parameters74,75 for the cation and the anion. In the [C4MIm][PF6] IL system at different temperatures (293, 323, and 353 K), 256 ion pairs (8192 atoms) were set in a cubic box and the periodic boundary condition was applied. The LennardJones interactions were calculated using a cutoff of 12 Å. The particle-mesh Ewald method76 was applied to compute Coulombic interactions using an electrostatic cutoff of 12 Å, a Fourier grid spacing of 2.0 Å, and a time step of 1.0 fs. Initially, each system was equilibrated under NPT conditions for 5 ns at each temperature with a target pressure of 1 atm. The temperature was kept constant with the Nosé-Hoover thermostat,77,78 while the pressure was controlled using the Parrinello-Rahman barostat.79 The time constants of temperature and pressure were set to be 0.8 and 1.0 ps, respectively. On the basis of the NPT simulation results, the length of the cubic box size was set to be 43.854, 44.028, and 44.299 Å for 293, 323, and 353K, respectively. Thereafter, we carried out NVT simulations for a 5 ns equilibration run followed by a 20 ns production run at each temperature. Simulation data were collected at 50 fs intervals during production runs. It is considered that the interionic vibrational spectrum of ILs includes the contribution from the interionic interaction among ion species and that important information on the motion of cation and anion in ILs can be provided with the DOS profile. Therefore, with MD simulation



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results, we computed velocity autocorrelation functions (VACFs), 〈vi(0)·vi(t)〉, where vi(t) is the velocity of the center of the mass of i-th ionic species, and obtained the vibrational DOSs with the Fourier transformation of VACFs.54,57,80 In this study, we performed the calculation of the time correlation of the total velocity which includes the velocities of all the atoms (cation and anion) at the different temperatures and also the time correlation functions were calculated for cation and anion species, at the different temperatures, respectively.

3. RESULTS 3.1. Temperature-Dependent Viscosities and Glass Transition Temperatures. Figure 2 shows the temperature dependence of η of [C4MIm][PF6], [C4MIm][DCA], and [C4MIm][NTf2], as examples. Most viscosities at various temperatures for the ILs studied here have been reported by other groups,81-85 being in good agreement with these reported values within experimental error. Note that the η data of [C4MIm][NCyF] are reported in this study for the first time. The temperature dependence of η shows non-Arrhenius behavior for each IL. The Vogel-Tammann-Fulcher (VTF) equation86-89 is commonly used to fit the temperature dependence of η of ILs90 and is given by

 DT0    T − T0 

η (T ) = η 0 exp

(1)

where T is the absolute temperature, η0 is the reference viscosity at which the exponential term approaches unity, D is a fragility related parameter, and T0 is the characteristic temperature at which η diverges. The collected data were fitted using the logarithmic form of eq 1, ln[η(T)] =


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ln(η0) + [DT0/(T–T0)], to properly treat the η data spanning several orders of magnitude. Each data set included a value of 1×1013 cP at the liquid’s Tg, according to the report by Angell and coworkers.90 Note that the η(T) fits of [C4MIm][OTf] and [C4MIm][NCyF] were obtained without the data at Tg because these ILs did not show glass transitions (described below). The corresponding fit parameters for η(T) of the studied ILs are summarized in Table 1. The Tg values for the studied ILs estimated in this study ranged from approximately 170 to 200 K. The [C4MIm][OTf] and [C4MIm][NCyF] crystallized during the first cooling cycle. To prevent these ILs from crystallizing, the fast cooling was performed at a scan rate of 10 K/min. However, these ILs crystallized again during the first cooling cycle, preventing the determination of their Tg values. Because the Tm of [C4MIm][NCyF] is 301 K, this IL existed in a supercooled state at room temperature in the present study. The values of Tm and Tg for the studied ILs are summarized in Table 1. The determined η (T) and Tg for the ILs enable the estimation of IL fragilities, which will be discussed in Section 4.1.

3.2 Temperature Dependent Low-Frequency Spectra Obtained by fs-RIKES Figure 3 displays the Kerr transients of [C4MIm][PF6] at 293, 308, 323, 338, and 353 K. The Kerr transients show instantaneous responses due to electronic responses at t = 0 and subsequent strong beats and overdamped decays arising from nuclear responses. The Kerr transients were normalized relative to the intensity of the electronic response at t = 0. Figure 3 reveals that the relative intensities of the nuclear responses in relation to the electronic responses increase with increasing temperature. This temperature-dependent behavior was confirmed for



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all the studied ILs regardless of their counter anions (see Supporting Information). The Kerr transients after 3 ps were fitted by a biexponential function:  t   t  a 0 + a1 exp −  + a 2 exp −   τ1  τ2

(2)

where a0 is the offset parameter reflecting the contribution of the slower relaxation component, a1 and a2 are the amplitudes, and τ1 and τ2 are the relaxation time constants for the respective components. The above time constants were almost independent of temperature within the experimental and fitting errors. For example, the fast and slow time constants for [C4MIm][PF6] are 1.46 ± 0.20 and 6.12 ± 1.01 ps, respectively. The offset parameter a0 increases with increasing temperature. However, in the case of molecular liquids (1-methylimidazole and propionitirile), the Kerr transients decay faster, and both the fast and slow time constants become faster with increasing temperature overall. No overdamped relaxation decay was observed for CCl4 in this temperature range because of its spherical top (tetrahedral) structure.

The

biexponential fit parameters for all the Kerr transients measured in this study, except for the transients for the case of CCl4, are summarized in Supporting Information. The Kerr transients were analyzed using the Fourier transform deconvolution procedure established by McMorrow and Lotshaw.91 Figure 4 shows the Fourier transform Kerr spectra of (a) [C4MIm][BF4], (b) [C4MIm][DCA], and (c) [C4MIm][NTf2] in the frequency range of 0 – 200 cm−1. The IL spectral line shapes at 293 K in this study are similar to those previously reported at room temperature.51,56,68,92 In addition to the above ILs shown in Figure 4, the lowfrequency spectra of all the ILs show a similar temperature dependence, although the line shape of each spectrum is different. That is, the spectral intensity in the low-frequency region below


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~50 cm−1 increases, and that in the high-frequency region above ~70 cm−1 decreases (or, in other words, the spectral density at the high frequency region shifts to the lower frequency side) with rising temperature. A similar temperature dependence was observed for the low-frequency spectra of other imidazolium-based ILs with the [NTf2]− anion reported by Quitevis and coworkers64,65 and our gorup.67 It is also noticeable from Figure 4 (actually Figures 5 and 6 as well, vide infra) that there is an isoscatteing point in the spectrum of each IL. An isosbestic point in an absorption spectrum appears when a perturbation (temperature in this study) affects two components or states in the spectrum. Thus, it could think that the results of the temperature dependent features of the IL spectra are attributed to the two distinguishable temperature dependent intermolecular vibrational motions. However, the Kerr spectra in this study are actually normalized at the electronic response in the time domain data and thus the Kerr spectral intensity is relative to the electronic response intensity but not absolute.

Furthermore, it is rather natural to think that two

intermolecular vibrational motions in liquids are coupled/interacting. Therefore, it is not clear whether the isoscatteing point observed in this study is real or artifact and thus we will not discuss this feature in this study. To determine the effect of the length of the imidazolium cation alkyl group, Figure 5 compares the temperature-dependent low-frequency spectra of (a) [C4MIm][PF6], (b) [C6MIm][PF6], and (c) [C7MIm][PF6]. The differences in the line shapes of the above spectra for the 1-alkyl-3-methylimidazolium hexafluorophosphates at room temperature shown in Figure 5 have been previously observed and attributed to the different frequencies of the intramolecular vibrational mode (bending of the alkyl group and imidazolium ring).51 Figure 5 reveals that the same temperature dependence was observed for the spectra of the three ILs, namely an increase



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of the spectral intensity in the low-frequency region below ~50 cm-1 and a decrease in the highfrequency region above ~70 cm-1. Figure 6 shows the temperature-dependent spectra of (a) 1-methylimidazole, (b) propionitrile, and (c) CCl4. Note that no spectrum is provided for CCl4 at 353 K because its boiling point at ambient pressure is 350 K.93 We found that the temperature dependence of the spectra of 1-methylimidazole was very similar to that of the ILs studied herein. However, propionitrile and CCl4 showed a clearly different temperature dependence.

Namely, the

corresponding spectral intensity in the low-frequency region below 50 cm−1 increased, similarly to the case of ILs and 1-methylimidazole, whereas the intensity in the high-frequency region above 70 cm−1 was largely unchanged. We further performed line shape analysis for the low-frequency Kerr spectra in order to discuss the temperature dependence quantitatively. The fitting functions used in this study are the sum of Ohmic function94 and antisymmetrized Gaussian functions.95 The Ohmic function is the simplified function of the Bucaro-Litovitz function with the exponent of the frequency term is 1,94 and it is also used to fit low-frequency Kerr spectra, instead of the Ohmic function. We used the Ohmic function in this study, because the number of parameter in the Ohmic function is smaller than the Bucaro-Litovitz function and the parameter of the exponent of the frequency (frequency dependent term) is often not stably obtained (or changeable) by fits.96 Lorentzian functions were also used when clear intramolecular vibrational bands were observed in the lowfrequency spectra. The details of the line shape analysis have been described in previous reports.10,42,43 On the basis of the above analysis, we estimated the first moments M1, which are the characteristic frequencies of the intermolecular vibrational bands in great major component, defined as


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M 1 = ∫ ωI (ω ) dω / ∫ I (ω ) dω

(3)

where I(ω) is the frequency-dependent intensity of a spectrum that does not contain the contributions of picosecond overdamped relaxation and clear intramolecular vibrations. The fit parameters and the values of M1 for the studied ILs and molecular liquids are summarized in Supporting Information. It is sometimes discussed each fit component obtained by line shape analysis for lowfrequency spectra of liquids/solutions.

However, M1 is used to discuss the temperature

dependent spectral features in the ILs in this study. This is because, it does not always mean that each function obtained by line shape analysis corresponds to an intermolecular vibrational mode. It is known from MD simulations that orientational and translational vibrational motions in liquids are strongly overlapped and coupled.97-101 The cross-term, which is a kind of coupling motion, often appears in spectrum with a negative amplitude that is hard to characterize by line shape analysis. Furthermore, two component systems, such as binary mixtures102-104 and ILs,5355,57,59

are even more complicated. Therefore, we focus here on the general or overall feature of

the temperature dependent low-frequency spectra of the ILs. Figure 7 shows the temperature dependence of M1 for the ILs with (a) spherical top anions, (b) less symmetric anions, (c) anions having conformers, and (d) cations with different alkyl group length. For comparisons, the M1 plots for 1-methylimidazole are also shown in the above figure, and the linear fits are also displayed by solid lines. The slopes obtained from linear fits are similar (approximately −0.056 ± 0.007 cm−1/K), except for [C4MIm][SCN] and [C4MIm][DCA]. This contrasts with the viscosities and thermal properties that significantly depend on the cation-anion combinations (Figure 2 and Table 1). Moreover, 1-methylimidazole



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also shows a similar slope of −0.057 cm−1/K. The slopes obtained for the M1 temperature dependence of the studied ILs and molecular liquids are summarized in Table 2.

3.3. Temperature-Dependent DOS Spectrum of [C4MIm][PF6] Calculated by MD Simulations. An advantage of MD simulation is its capability of the decomposition analysis for the complicated and overlapped motions. In the case of most common ILs, these motions include contributions at least two components, namely the cation and the anion. Thus, it is very helpful to understand the motions and spectral profiles of ionic species in ILs.

Even though the

calculated DOSs do not directly correspond to the experimental Kerr spectra, the major purpose in this study of understanding the temperature dependence of spectra is to determine the nature of temperature-sensitive species in ILs and their motion frequencies. Thus, computing the DOS spectra is considered to be appropriate for the present study. Figure 8 compares the DOS spectra of (a) [C4MIm][PF6] at 293, 323, and 353 K, which are decomposed into the contributions of (b) [C4MIm]+ and (c) [PF6]– to see the effects of each constituent ion.

The DOS profiles of [C4MIm][PF6], [C4MIm]+, and [PF6]– are in good

agreement with the one calculated previously for these species.57 Note that the DOS spectra are not directly related to the Kerr spectra, because the latter are based on this fs-RIKES selection rule (depolarized Raman signal), while the former feature equal contributions of all vibrational states existing within the frequency region (Fourier transform power spectrum of the time correlation function of the velocity of ionic species).

Nonetheless, the comparison of

temperature-dependent low-frequency Kerr spectra with the calculated DOS spectra is useful.


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As shown in Figure 8, the spectral intensities in the low-frequency region below 50 cm-1 increase with increasing temperature for both the cation and anions (and thus for the entire IL). Moreover, the shoulder at ~80 cm-1 in the cation DOS spectrum for the cation shifts to the lowerfrequency side with increasing temperature. The temperature-dependent spectral features of the [C4MIm][PF6] DOS spectra qualitatively agree with the results of the fs-RIKES experiments, which will be further discussed in Section 4.2.

4. DISCCUSION 4.1. Fragilities of Ionic Liquids. Fragility is an important physical parameter for glass-forming liquids, being related to the non-Arrhenius behavior of the temperature dependence of the viscosity.105,106 Many ILs are known to exhibit a supercooled state. Therefore, discussing the fragilities of the studied ILs is desirable before describing the temperature dependence of the intermolecular vibrational dynamics. Figure 9 shows the Angell plots90 for the studied ILs with the exception of [C4MIm][OTf] and [C4MIm][NCyF], which do not exhibit glass transitions. To see the big picture for the ILs, the plots for the two extreme cases, SiO2107,108 and o-terphenyl,109-111 which show strong and fragile features, respectively, are also shown. Figure 9a reveals that the studied ILs are rather fragile compared to SiO2 but stronger than o-terphenyl. Furthermore, the ILs roughly show a single curve in large scale plots (Figure 9a). However, careful examination of the magnified plots in Figure 9b shows that the curve is dependent on the nature of IL anions. The obtained



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data indicate that ILs containing spherical top anions are stronger than the ones containing other anions (less symmetric and flexible ones). The IL fragility features in Figure 9a are related to the VTF fit parameter D. Because the liquid fragility is proportional to D-1, the results of this study are reasonable, indicating that more compact (smaller) anions make the corresponding imidazolium-based ILs stronger. Figure 9c compares the plots for [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6] to illustrate the effect of the alkyl group attached to the cation. The differences between these ILs are not large, with [C7MIm][PF6] being the strongest IL. The results of VTF fits in Table 1 show that the D parameter increases with the length of the IL alkyl group, indicating that longer alkyl groups result in stronger liquids because of stronger van der Waals interactions. Previously, Triolo and coworkers studied the small to wide angle X-ray diffraction patterns of ILs and found that the intensity in the low Q peak (attributed to the segregation structure) for imidazoliumbased ILs gains intensity and shifts to lower Q for longer alkyl groups of the imidazolium cation.112 These results indicate that the micro-segregation structures in ILs are clearer in the imidazolium-based ILs with cations having longer alkyl groups. Because the fragility parameter is related to the dynamic heterogeneity,106 the present results might indicate a relation between the fragility and the dynamic heterogeneity of ILs.

4.2. Temperature Dependence of Low-Frequency Spectra. Prior to discussing the results of this study, we summarize some previous reports on the temperature dependence of the low-frequency spectra of imidazolium-based ILs studied by fsRIKES. Quitevis and coworkers reported the temperature dependence of the normalized low-


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frequency spectra characterized by fs-RIKES for 1-methyl-3-pentylimidazolium ([C5MIm])based ILs with [NTf2]−,64 bromide (Br−), and [PF6].65 The authors showed that the spectra of [C5MIm]Br and [C5MIm][PF6] were temperature-independent.

However, the spectrum of

[C5MIm][NTf2] was temperature-dependent, with the spectral intensity in the high-frequency region above 60 cm-1 is decreasing with rising temperature.

Note that normalization is

performed relative to the highest peak intensity in the low-frequency spectrum, with the contribution of overdamped relaxation subtracted from the entire spectrum.

The authors

attributed the results to the difference in the liquid natures of the ILs, with [C5MIm]Br and [C5MIm][PF6] having a larger solid-like nature, while [C5MIm][NTf2] having a more liquid-like one. Recently, we studied the temperature dependence of the low-frequency spectra of [NTf2]– salts with monocations (1-methyl-3-propylimidazolium and 1-hexyl-3-methylimidazolium) and dications

(1,6-bis(3-methylimidazolium-1-yl)hexane

and

1,12-bis(3-methylimidazolium-1-

yl)dodecane) by fs-RIKES.67 The results showed that the intensity in the low-frequency region below 20 cm−1 increases with increasing temperature, whereas the high-frequency component with a peak at approximately 80 cm−1 shifts to the lower frequency side and the spectral intensity in the high-frequency region above ~70 cm-1 slightly decreases with rising temperature for both the monocationic and dicationic ILs. At a first glance, the results of Quitevis and coworkers and our group seem to be different. However, the temperature-dependent spectral features are quite similar for the normalized low-frequency spectra. It should be reminded that the Kerr transients are normalized with respect to the intensity at t = 0 before Fourier transform deconvolution analysis. Therefore, the normalized low-frequency Kerr spectra are actually renormalized. In this study, we discuss the low-frequency spectra without renormalization, as shown in Figures 4 – 6. Essentially, the temperature dependence behaviors of imidazolium-based ILs with the



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[NTf2]− anion obtained by Quitevis et al. and our group are in good agreement. On the other hand, the intriguing temperature-dependent spectral features of [C5MIm]Br and [C5MIm][PF6] seem to be different from the results of [C4MIm][PF6] in this study. The temperature-dependent spectral features of various ILs are discussed below.

4.2.1. Effects of Various Anions. As shown in Figure 4 and Supporting Information, the low-frequency Kerr spectra for all the studied [C4MIm]+-based ILs with ten different anions are temperature-dependent, as well as the three molecular liquids. As mentioned in Section 3.2, the temperature-dependent spectral features of all the ILs are similar, also resembling those of 1-methylimidazole, but being clearly different from those of propionitrile and CCl4. These results indicate that the temperature dependence of the low-frequency spectra of the ILs is largely affected by the imidazolium ring. On the basis of this assumption, we consider the mechanism of this temperature dependence of the low-frequency spectra of the imidazolium-based ILs. The low-frequency spectral shapes of the ILs studied here are bimodal or trapezoidal, being common features of ILs based on aromatic cations with simple alkyl groups.50 Indeed, not only aromatic cation-based ILs, but also most aromatic molecular liquids show broad line shapes in their low-frequency spectra.8,10,113 According to an MD simulation of liquid benzene,97 the frequency region above 60 cm-1 in the low-frequency spectral bands is assigned to the librational motion of the aromatic ring. According to a recent study combining fs-RIKES experiments and MD simulations for liquid benzene at various temperatures,114 the high-frequency portion of the low-frequency spectrum of liquid benzene (described to an anti-symmetrized Gaussian function) shifts to the low-frequency


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side with increasing temperature, and this result is qualitatively reproduced by the MD simulation. These results show that density (free volume), and not temperature (purely thermal effect), is the dominant factor. Ratajska-Gadomska also simulated the temperature-dependent Kerr spectrum of liquid benzene based on the quasi-crystalline model, and a red shift of the highfrequency component was observed with increasing temperature.115 As mentioned in Section 3.2, the spectral intensity in the high-frequency region of the intermolecular vibrational spectra of the studied ILs decreases with increasing temperature, which has also been reported for some molecular liquids.113,116-118 These results indicate that the librational motion of the imidazolium ring becomes slower with rising temperature, probably due to the increase of the IL free volumes that increase the libration displacement magnitude and weakens the intermolecular interactions (thus making the librational motion slower).

Note that unlike the diffusion process the

vibrational motion (e.g., harmonic oscillator) becomes slower (or lower in the frequency region) with weakening the intermolecular interaction. On the other hand, several different types of molecular motions, such as translational motion, rotational motion, and their cross-term, contribute to the spectral density in the lowfrequency region below ~50 cm-1.97 Moreover, the crossover process from the intermolecular vibrational motions to the collective structural relaxation also contribute to this frequency region.106,119 As seen in Figure 4 and Supporting Information, the spectral intensity in the lowfrequency region of the IL low-frequency spectra increases with increasing temperature, attributed to the activation (less hindrance) of translational motions of ions with increasing temperature that gives the caging effect being weaker.

In fact, some studies describe the

influences of the translational motions on the low-frequency region of the intermolecular vibrational band.16-18,56,57 The spectra of [C4MIm]-based ILs with [XF6]− anions showed that the



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intensities in the frequency region below 20 cm−1 decrease when [PF6]− is substituted with heavier anions, [AsF6]− and [SbF6]−.56

This is attributed to the lesser activation of the

interaction-induced motions in [C4MIm][AsF6] and [C4MIm][SbF6] than [C4MIm][PF6].56,57 Another helpful case is the comparison of the spectra of polymer, oligomer, and monomer solutions.

It is reported that the spectral intensities in the low-frequency region of the

intermolecular vibrational spectra of polymer and oligomer solutions are lower than those of the corresponding monomer solutions, which is consistent with their mass.16-18 These results suggest that the intensity increases in the low-frequency region with rising temperature is accounted for by the activation of the interaction-induced (translational) motions. To provide more details for the temperature-dependent spectral features of the lowfrequency spectra, Figure 10 shows the difference spectra for 308, 323, 338, and 353 K relative to the spectra at 293 K for (a) [C4MIm][BF4], (b) [C4MIm][NTf2], (c) [C4MIm][SCN], (d) [C4MIm][DCA], (e) 1-methylimidazole, and (f) propionitrile. The difference spectra for the other ILs studied here are provided in Supporting Information.

[C4MIm][BF4] and

[C4MIm][NTf2] display representative difference spectra, showing a large increase in the lowfrequency (~5 cm−1) region, a slight increase in the intermediate-frequency (~50 cm−1) region, and a decrease in the high-frequency (~110 cm−1) region. The temperature dependence of the difference spectra for 1-methylimidazole is close to that for the ILs, indicating that the librational motion activity decreases and the translational motion and/or the cross-term increases with rising temperature.

Thus, the effect of temperature on the low-frequency Kerr spectra of the

imidazolium-based ILs is largely attributed to the motion of the imidazolium ring. However, two specific features are found for the temperature dependence of the difference spectra upon closer examination.


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First, the ILs show a steeper intensity change in the ~5 cm−1 frequency region with increasing temperature than 1-methylimidazole. Moreover, the changes observed for ILs are more similar to those detected for the smaller propionitrile than for the larger and planer 1methylimidazole. This indicates that the anions also contribute to the temperature-dependent spectrum in the ~5 cm−1 frequency region. One can think that the spherical top anions contribute to the Kerr spectra very weakly. As shown in Figure 6, a low-frequency spectral band with a peak at ~15 cm-1 without the strong spectral density in the high frequency region is observed in liquid CCl4. It is thus plausible that the temperature effect in the low-frequency (~5 cm−1) region features the contributions of both the IL cations and anions. Next, comparing the difference spectra of [C4MIm][BF4] and [C4MIm][NTf2] with those of [C4MIm][SCN] and [C4MIm][DCA], a clear dissimilarity is found in the intermediatefrequency (~50 cm−1) region. This would be attributed to the librational motions of the [SCN]− and [DCA]− anions. These anions are expected to contribute to the Kerr spectra in large because they are not spherical top symmetry. Thus, their contributions might lie in the intermediatefrequency region, and shift to lower frequency with increasing temperature, similarly to the librational motions of the imidazolium ring. Notably, many nonaromatic aprotic molecular liquids show large spectral densities in the frequency range of approximately 5 – 80 cm-1, which is lower than aromatic molecular liquids.10 Accordingly, it is not surprising that the decrease of the spectral intensities with increasing temperature in the intermediate-frequency region is observed for the imidazolium-based ILs with the non-spherical top anions that probably exhibit strong Kerr activities. On the other hand, it is surprising that the ILs with the anions having two stable conformers, i.e., [NF2]−, [NTf2]−, and [NPf2]−, show the temperature dependence of M1 (slope) 23 Environment ACS Paragon Plus


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(Figure 7 and Table 2) and the difference spectra (Figure 10) similar to those of ILs with spherical top anions and 1-methylimidazole. This could be caused by the frequency of the anion vibrational motions. Because the librational motions of these anions are in the low-frequency region of ~20 cm-1,51,68 the dissimilarity in the difference spectra caused by anions should also appear in this frequency region, but not in the intermediate-frequency region of ~50 cm-1. Furthermore, the M1 (see the definition of eq. 3) should be more sensitive to the spectral change in the high-frequency region than in the low-frequency region. As a result, the temperature dependence of M1 and the difference spectra for [C4MIm][NF2], [C4MIm][NTf2], and [C4MIm][NPf2] are similar to those of the ILs with spherical top anions and 1-methylimidazole.

4.2.2. Effect of Alkyl Groups Attached to Imidazolium Cation. As seen in section 4.2.1, we clearly observed the temperature dependent spectral features of the imidazolium-based ILs including [C4MIm][PF6]. It seems that the results are different from the results for [C5MIm][PF6] reported by Xiao et al.65 One can think that different alkyl groups attached to the imidazolium cation result in different temperature dependence of the lowfrequency spectra of ILs. As shown in Figure 5, however, the temperature-dependent spectral features for [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6] are quite similar. To be more specific, Figure 11 shows the difference spectra for 308, 323, 338, 353 K in relation to those at 293 K for (a) [C4MIm][PF6], (b) [C6MIm][PF6], and (c) [C7MIm][PF6]. The above figure reveals that all the ILs show a similar temperature dependence of the difference spectra, with the intensity in the high-frequency region above ~80 cm-1 decreasing, and the intensity in the lowfrequency region below ~70 cm-1 increasing with rising temperature. Accordingly, we conclude


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from the present results that the alkyl groups of the imidazolium cations do not greatly influence the temperature dependence of the low-frequency spectra. As mentioned above (introduction in Section 4), the temperature-independent lowfrequency Kerr spectrum for [C5MIm][PF6] was previously reported.65 Although the reported sample was not exactly identical to the ILs used in the present study, the results for [C5MIm][PF6] somewhat disagree with those obtained for [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6]. One might think that [C5MIm][PF6] is an exception in the series of 1-alkyl-3methylimidazolium hexafluorophosphates. However, several possible reasons would explain this inconsistency. The first one is the difference in the signal-to-noise ratios between the spectra of [C5MIm][PF6] and the other ILs, [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6]. The signal-to-noise ratio in the spectra of [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6] is higher than that of [C5MIm][PF6], explaining the small differences in their spectra, such as those in the frequency region above 80 cm-1 for [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6] (Figure 5). The second reason is the renormalization of the [C5MIm][PF6] spectra.65 If we consider the renormalized low-frequency spectra of [C6MIm][PF6] and [C7MIm][PF6], the spectral intensity difference in the low-frequency region below 50 cm-1, which is clearly observed in the nonrenormalized spectra in Figure 5, is expected to be small. Finally, the effect of the overdamped relaxation component provides the third reason to explain the observed behavior.

The

overdamped relaxation components over several picoseconds are excluded in the spectra of [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6], while the subpicosecond components are removed in the case of [C5MIm][PF6].65 These differences between the spectra of [C5MIm][PF6] and the other ILs, [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6], probably lead to the different temperature-dependent spectral features.

We therefore conclude that the



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renormalization of spectra measured by fs-RIKES introduces ambiguity into their temperature sensitivity, and the inconsistency in the temperature dependence of the low-frequency spectra between [C5MIm][PF6] and [C4MIm][PF6], [C6MIm][PF6], and [C7MIm][PF6] is probably due to these three possibilities.

4.2.3. Comparison of Experimental Kerr Spectra and Calculated DOS Spectra. In contrast to sharp intramolecular vibrational modes, assigning broadened low-frequency spectra of liquids is not straightforward, because the low-frequency band includes different kinds of motions, such as translational and reorientational motions and their coupling motions. ILs show an even more complicated behavior, being composed of different species, i.e., cations and anions. MD simulations are good at clarifying them. Herein, we compare the temperaturedependent Kerr spectrum of [C4MIm][PF6] with its temperature-dependent DOS spectrum to elucidate the origin of the temperature-sensitive features in the low-frequency Kerr spectra. Comparison of the Kerr spectra (Figure 5a) and DOS spectra (Figure 8a) of [C4MIm][PF6] shows that their line shapes are quite different. As mentioned above, this likely arises from the difference in the selection rules between the spectra.

Considering the

polarizability anisotropies of the cation and anion,56 the contribution of the cation to the intensity in the Kerr spectrum is expected to be much larger than that of the anion. It is thus reasonable that the Kerr spectrum (Figure 5a) of [C4MIm][PF6] is more similar to the DOS spectrum of [C4MIm]+ (Figure 8b) rather than to that of [C4MIm][PF6] (Figure 8a) or [PF6]− (Figure 8c).


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The focus of this study is the temperature dependence of spectra. The DOS spectra of [C4MIm]+ and [PF6]− in Figure 8 show that the intensities in the ~30 cm-1 frequency region increases with rising temperature in both cases. As a result, the intensity in the ~30 cm-1 region for the entire [C4MIm][PF6] DOS spectrum also increases at higher temperatures. This behavior agrees well with the temperature-dependent feature in the low-frequency region of the Kerr spectrum shown in Figure 5. Therefore, the MD simulations indicate that the temperaturedependent spectral change in the low-frequency region is actually attributable to both the cation and anion species. On the other hand, the temperature-related changes in the high-frequency region (> 70 cm-1) of the Kerr spectrum are quite similar to those of the DOS spectrum of [C4MIm]+, however, this feature is not reproduced in the DOS spectrum of [PF6]−. This is in good agreement with our assignment mentioned above. Accordingly, the temperature-dependent features of the low-frequency Kerr spectrum of [C4MIm][PF6] are well accounted for by the results of MD simulations.

4.3. Fragility and Temperature Dependence of Low-Frequency Spectrum. In the last section, we would like to make a brief comment on the relation between the temperature dependence of the low-frequency spectra and the fragilities of ILs, because we believe that it is important to understand/determine the effect of intermolecular vibrations on the nature of liquid fragility. The temperature dependence degree of the low-frequency Kerr spectra (Table 2) and the values of the fragility-related parameter D (Table 1) for the studied ILs are hard to correlate. For example, ILs with spherical top anions show larger D values than the other ones, but the temperature dependence degrees of their low-frequency Kerr spectra are quite



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similar to those of several ILs with flexible anions ([N(SO2CnF2n+1)2]−: n = 0 – 2), [NCyF]−, and [OTf]−, and even to that of a molecular liquid, 1-methylimidazole. The D values seem to be dependent on the alkyl group length for the imidazolium cation in 1-alkyl-3-methylimidazolium hexafluorophosphates, but not on the temperature dependence degree of the low-frequency Kerr spectra. This suggests that caging motion such as libration does not directly influence the fragility of ILs to a large extent. Slower relaxation processes such as so-called α-relaxation (structural relaxation) that represent motions on a larger space (length) scale motion than the intermolecular vibration might be a more appropriate for comparison with the fragility parameter. In fact, Ribeiro pointed out that the temperature-dependent intensity of the quasi-elastic scattering, which is mainly due to the contribution of the relaxational component rather than to that of intermolecular vibrations, is related to the fragility parameter for some ILs.33

5 CONCLUSIONS In this study, we investigated the temperature dependence of the low-frequency spectra for imidazolium-based ILs with ten different anions obtained by fs-RIKES.

The spectral

intensities in the frequency region below 20 cm−1 increase with increasing temperature due to the activation of the translational vibrational motions of IL ions at higher thermal energies. In addition, the spectral intensity in the high-frequency region above 80 cm−1 decrease with rising temperature, attributed to the slower librational motion of the imidazolium ring due to the larger free volume and the weaker intermolecular interaction at higher temperature. The results of the DOS spectra calculated for [C4MIm][PF6] by MD simulation support the assignments of the temperature-dependent spectral features for the imidazolium-based ILs.


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The difference between the spectra of ILs at certain temperatures and at 293 K revealed the temperature-dependent spectral features in greater detail. The temperature dependence of the difference spectra for 1-methylimidazole is rather similar to those for the ILs, suggesting that the effect of temperature on the low-frequency Kerr spectra of the imidazolium-based ILs is largely due to the motions of the imidazolium cation. However, [C4MIm][SCN] and [C4MIm][DCA] showed partially different from those of other ILs and 1-methylimidazole, with the difference spectral density in the intermediate-frequency region (~50 cm-1) of these two ILs being distinguishable from that of other ILs, attributed to the librational motions of [SCN]− and [DCA]− anions. In fact, the temperature dependence degrees of the low-frequency spectra (that is the slope for the first moment of the spectrum to the temperature) of the ILs are in good agreement with the features of the difference spectra. The temperature dependence of viscosities and the glass transition temperatures of the ILs were also estimated. On the basis of these physical properties and VTF fits, the fragilityrelated parameters (D parameter for the VTF fit, eq. 1) of the ILs were determined. The D parameter was particularly large for ILs with spherical top anions, as compared to those with other anions. We also found that the D value increases for the longer alkyl groups of the IL imidazolium cations. On the other hand, there is no clear direct relationship between D and the temperature dependence of M1 for the low-frequency spectrum of the studied ILs.

Supporting Information Details of synthesis procedures of ILs synthesized in this study, water contents of ILs used in this study, temperature dependent Kerr transients, spectra, and difference spectra, and lists of fit



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parameters for the Kerr transients and the Kerr spectra of the ILs. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (15K05377, HS).

We are grateful to Mr.

Motoyasu Fujiwara (the Institute for Molecular Science) for his kind help with the DSC measurements at the Institute for Molecular Science. We also thank the Institute for Molecular Science for its generous support in conducting DSC measurements and the Research Center for Computational Science, Okazaki, Japan, for performing MD simulations.


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TABLE 1. Melting Points (Tm), Glass Transition Temperatures (Tg), and VTF Fit Parameters for ILs.

a

IL

Tm (K)

Tg (K)

η0 (cP)

D

T0 (K)

[C4MIm][PF6]

283

192

0.0289

8.766

152.2

[C4MIm][BF4]

n.o.b

179

0.0205

9.823

138.7

[C4MIm]I

n.o.b

197

0.0030

14.60

139.9

[C4MIm][OTf]a

285

n.o.b

(0.0838)

(6.154)

(158.0)

[C4MIm][SCN]

n.o.b

181

0.0351

7.925

146.2

[C4MIm][DCA]

n.o.b

174

0.0902

5.874

147.3

[C4MIm][NCyF]a

301

n.o.b

(0.0981)

(5.547)

(171.3)

[C4MIm][NF2]

n.o.b

172

0.1216

5.818

145.6

[C4MIm][NTf2]

269

183

0.0986

5.665

155.7

[C4MIm][NPf2]

n.o.b

186

0.0456

7.487

151.6

[C6MIm][PF6]

n.o.b

198

0.0200

9.351

155.1

[C7MIm][PF6]

n.o.b

197

0.0146

10.37

151.1

VTF fit parameters were obtained without Tg data. bn.o. denotes not observed.



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TABLE 2. Slopes estimated from Linear Fits for the First Moments M1 to Temperature for ILs and Molecular Liquids. Spherical Top Anion Sample

Slope (cm−1/K)

Lower Symmetric Anion Sample

Anion having Conformers

Molecular Liquid

Slope (cm−1/K)

Sample

Slope (cm−1/K)

Sample

Slope (cm−1/K)

[C4MIm][PF6]

−0.058

[C4MIm][OTf]

−0.062

[C4MIm][NF2]

−0.056

1-Methylimidazole

−0.057

[C4MIm][BF4]

−0.056

[C4MIm][SCN]

−0.041

[C4MIm][NTf2]

−0.050

Propionitrile

−0.104

[C4MIm]I

−0.053

[C4MIm][DCA]

−0.037

[C4MIm][NPf2]

−0.048

CCl4

−0.056

[C6MIm][PF6]

−0.055

[C4MIm][NCyF]

−0.065

[C7MIm][PF6]

−0.054


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Figure Captions. Figure 1. Chemical formulas of cations and anions of ILs used in this study. Figure 2.

Temperature-dependent shear viscosities for [C4MIm][PF6] (red circles),

[C4MIm][DCA] (blue squares), and [C4MIm][NTf2] (green triangles). The solid lines denote the VTF fits to eq. 1 for each IL. Figure 3. Kerr transients for [C4MIm][PF6] at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red). Figure 4.

Low-frequency Kerr spectra in the frequency range of 0 – 200 cm−1 for (a)

[C4MIm][BF4], (b) [C4MIm][DCA], and (c) [C4MIm][NTf2] at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red). Figure 5.


[C4MIm][PF6], (b) [C6MIm][PF6], and (c) [C7MIm][PF6] at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red). Figure 6. Low-frequency Kerr spectra in the frequency range of 0 – 200 cm−1 for (a) 1methylimidazole, (b) propionitrile, and (c) CCl4 at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red). Figure 7. Plots of first moment vs. temperature for ILs with (a) spherical top anions, (b) less symmetric anions, (c) anions having conformers and (d) cations with different alkyl chain lengths.

Plots for 1-methylimidazole (black crosses) are included in all the figures for

comparison. Linear fits are shown by solid lines.



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Figure 8. Calculated temperature-dependent DOS spectra for (a) [C4MIm][PF6], (b) [C4MIm]+, and (c) [PF6]–.

Black, green, and red lines denote the spectra at 293, 323, and 353 K,

respectively. Figure 9. (a) Full scale plots of ln(η) vs. Tg/T. Expanded scale plots for (b) [C4MIm]+–based ILs with various anions and (c) [PF6]– salts of 1-alkyl-3-methylimidazolium cations. Filled circles: SiO2;107,108 filled squares: o-terphenyl;109-111 red circles: [C4MIm][PF6]; red squares: [C4MIm][BF4]; red triangles: [C4MIm]I; blue circles: [C4MIm][SCN]; blue squares: [C4MIm][DCA]; green circles: [C4MIm][NF2]; green squares: [C4MIm][NTf2]; green triangles: [C4MIm][NPf2]; red pluses: [C6MIm][PF6]; red crosses: [C7MIm][PF6]. Difference Kerr spectra of (a) [C4MIm][BF4], (b) [C4MIm][NTf2], (c)

Figure 10.

[C4MIm][SCN], (d) [C4MIm][DCA], (e) 1-methylimidazole, and (f) propionitrile for the spectra at 308 (blue), 323 (green), 338 (yellow), and 353 (red) K relative to that at 293 K. Figure 11.

Difference Kerr spectra of (a) [C4MIm][PF6], (b) [C6MIm][PF6], and (c)

[C7MIm][PF6] for the spectra at 308 (blue), 323 (green), 338 (yellow), and 353 (red) K relative to that at 293 K.


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Cations

N

CnH2n+1

N

n = 4: [C4MIm]+ n = 6: [C6MIm]+ n = 7: [C7MIm]+

Anions F F F P F F F [PF6]-

F B F F F [BF4]-

O F3 C S O O

N C S

[OTf]-

[SCN]-

O O N S S F F O O [NF2]-

N

I

C

N

C

[DCA]-

O O N S S F3 C CF3 O O [NTf2]-

N

O O N O S S O F2 C CF2 C F2 [NCyF] -

O O N S S C2F5 C2F5 O O [NPf2]-

Figure 1. Chemical formulas of cations and anions of ILs used in this study.



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Figure 2.

Page 50 of 61

Temperature-dependent shear viscosities for [C4MIm][PF6] (red circles),

[C4MIm][DCA] (blue squares), and [C4MIm][NTf2] (green triangles). The solid lines denote the VTF fits to eq. 1 for each IL.


Page 51 of 61

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Figure 3. Kerr transients for [C4MIm][PF6] at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red).



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Figure 4.

Page 52 of 61


[C4MIm][BF4], (b) [C4MIm][DCA], and (c) [C4MIm][NTf2] at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red).


Page 53 of 61

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Figure 5.


[C4MIm][PF6], (b) [C6MIm][PF6], and (c) [C7MIm][PF6] at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red).



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Figure 6. Low-frequency Kerr spectra in the frequency range of 0 – 200 cm−1 for (a) 1methylimidazole, (b) propionitrile, and (c) CCl4 at 293 (black), 308 (blue), 323 (green), 338 (yellow), and 353 K (red).


Page 54 of 61

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Figure 7. Plots of first moment vs. temperature for ILs with (a) spherical top anions, (b) less symmetric anions, (c) anions having conformers and (d) cations with different alkyl chain lengths.

Plots for 1-methylimidazole (black crosses) are included in all the figures for

comparison. Linear fits are shown by solid lines.



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Page 56 of 61

Figure 8. Calculated temperature-dependent DOS spectra for (a) [C4MIm][PF6], (b) [C4MIm]+, and (c) [PF6]–.

Black, green, and red lines denote the spectra at 293, 323, and 353 K,

respectively.


Page 57 of 61

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Figure 9. (a) Full scale plots of ln(η) vs. Tg/T. Expanded scale plots for (b) [C4MIm]+–based ILs with various anions and (c) [PF6]– salts of 1-alkyl-3-methylimidazolium cations. Filled circles: SiO2;107,108 filled squares: o-terphenyl;109-111 red circles: [C4MIm][PF6]; red squares: [C4MIm][BF4]; red triangles: [C4MIm]I; blue circles: [C4MIm][SCN]; blue squares: [C4MIm][DCA]; green circles: [C4MIm][NF2]; green squares: [C4MIm][NTf2]; green triangles: [C4MIm][NPf2]; red pluses: [C6MIm][PF6]; red crosses: [C7MIm][PF6].



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Figure 10.

Page 58 of 61

Difference Kerr spectra of (a) [C4MIm][BF4], (b) [C4MIm][NTf2], (c)

[C4MIm][SCN], (d) [C4MIm][DCA], (e) 1-methylimidazole, and (f) propionitrile for the spectra at 308 (blue), 323 (green), 338 (yellow), and 353 (red) K relative to that at 293 K.


Page 59 of 61

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Figure 11.

Difference Kerr spectra of (a) [C4MIm][PF6], (b) [C6MIm][PF6], and (c)

[C7MIm][PF6] for the spectra at 308 (blue), 323 (green), 338 (yellow), and 353 (red) K relative to that at 293 K.



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Femtosecond Raman-Induced Kerr Effect Study of Temperature

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